Due to improved performances of computer systems, there are increased demands to perform theoretical chemistry computations including molecular orbit computations, molecular dynamics computations and the like in high-tech research, such as drug design. In order to perform the theoretical chemistry computation, it is necessary to use as input data not only the computation method but also the molecular structure of the computation target. In the high-tech research such as the drug design, it is often desirable to find a new molecular structure which can improve the effects the property of the matter, such as the polarizability, over the conventional molecular structure. Thus, there are increasing demands for constructing an arbitrary molecular structure. In order to satisfy such demands by improving the operation efficiency of the molecular structure construction, molecular design systems utilizing the graphic display function have been proposed in Japanese Laid-Open Patent Publications No. 5-174109 and No. 5-128218.
According to the molecular structure modifying techniques that have been proposed, there is a limit to the number of times an Undo operation and a Redo operation may be performed. The Undo operation returns the molecular structure after the modification back to the original molecular structure, and the Redo operation modifies the molecular structure again after the Undo operation. The Undo and Redo operations can only be performed once or, several times at the most. The number of times the Undo and Redo operations may be performed is limited because the storage capacity that needs to be used for such a modification is extremely large. For this reason, according to the conventional molecular structure modifying techniques, there was a problem in that it is difficult to easily perform an arbitrary modification on the molecular structure.